Microelectrochemical reactors increasingly have been studied for use in a variety of applications. In analytical applications, microelectrochemical reactors can provide for measurement of small quantities or concentrations of an analyte, as long as the analyte can undergo an electrochemical reaction. In power generation applications, microelectrochemical reactors can be used to construct batteries or fuel cells that are extremely small and light weight. In other applications, microelectrochemical reactors can be used to perform microchemical synthesis, to perform microfluidic pumping, or to pattern a surface.
For example, microelectrochemical sensors have been used for applications ranging from the detection of glucose by diabetes patients to the detection of small amounts of gas in liquid or atmospheric samples. Conventional gas sensing systems are typically based on amperometric Clark sensors having a chamber in which an electrolyte and microelectrodes are housed. A gas permeable membrane is used to separate the electrolyte and electrodes from the outside sample. The purpose of the membrane is to enhance the sensitivity and to shield the electrode from interfering species. However, the performance of these sensors can be limited by the diffusion of the gaseous analyte through both the membrane and the electrolyte. Typically, these sensor systems are less sensitive than a bare platinum electrode under a controlled environment.
In another example, fuel cells are under intense development as potential alternative energy sources that are lightweight and compact and that can provide high power densities. When oxygen is consumed by a fuel cell, the activity of the cathode where the oxygen is reduced can be particularly degraded by the concentration polarization associated with the mass transport resistance within the fuel cell. It is believed that minimizing this resistance can provide an improvement in power density operation of the fuel cell. In polymer electrolyte membrane (PEM) fuel cells, platinum is used as the cathode, and a perfluorosulfonic acid polymer membrane is the solid electrolyte. Although the polymer membrane can minimize the degradation of the cathode activity by suppressing the crossover of the anode fuel to the cathode, the lifetime of the PEM fuel cell typically is limited by the dehydration of the polymer membrane, due to the high temperatures developed during operation of the fuel cell. Even without the constraints of a membrane, the performance of a fuel cell typically will be limited by the amount of dissolved oxygen in the liquid electrolyte.
In another example, electrochemical reactions have been used to provide for pumping liquids within microfluidic systems for applications such as drug delivery, chemical blood analysis and flow cytometry. Pumping systems for microfluidic devices typically have been based on either electrokinetic or hydrodynamic principles. Electrokinetically induced pumping requires high voltages of approximately 30 kV, and the electrical currents generated can cause water electrolysis at the electrodes, resulting in significant Joule heating and generation of bubbles. Hydrodynamic pumping requires the use of auxiliary, macroscopic components such as syringe pumps, which are complicated to integrate with the microfluidic system and are not typically disposable. However, a microelectrochemical approach to microfluidic pumping involves the generation of a surface pressure gradient between two microelectrodes and requires only low voltages. The formation of a surface active species at one electrode and its consumption at the other electrode can provide sufficient surface pressure to move droplets of organic liquids through a microfluidic network. A disadvantage of the typical microelectrochemical approach is the requirement for a redox active surfactant in the system, which may be incompatible with the fluid of interest or with the surface properties of the network material.
It is thus desirable to provide microelectrochemical reactors having improved reactivity for the reactions occurring at the microelectrodes. It is also desirable to provide microelectrochemical devices with improved performance, such as sensors having improved sensitivity, fuel cells providing increased power and current densities, and microfluidic pumps having a simpler design and a more effective performance.